Changes for 4.19:

- Use extent maps to track pagecache page status instead of bufferhead
  state.
- Refactor pagecache read and write paths to use the new iomap library
  functions, which enable us to drop the old bufferhead code for
  pagesize == blocksize filesystems.
- Set up parallel per-block-per-page metadata to track subpage
  information that was tracked by buffer heads, which enables us to drop
  the old bufferhead code for pagesize > blocksize filesystems.
- Tie a deferred ops control structure to a transaction so that we can
  take advantage of an upper-level dfops without having to plumb pointer
  passing through the code.
- Refactor the deferred ops code to track deferred ops as part of the
  transaction structure (instead of as a separate data structure) so
  that we can simplify the scoping rules around defer_ops.
- Refactor twisty delwri buffer submission code to avoid deadlocks.
- Shorten and fix indenting problems in the scrub code.
- Detect obviously bad summary counts at mount and fix them.
- Directly associate deferred ops control structure with a transaction
  so that callers no longer have to manage it themselves.
- Remove a couple of IRIX-era inode macros.
- Remove the long-deprecated 'barrier' and 'nobarrier' mount options.
- Clean up the inode fork structure a bit.
- Check for bad fs summary counter values in the superblock.
- Reduce COW fork lookups during writeback.
- Refactor the deferred ops control structures into the transaction
  structure, thereby eliminating the need for transaction users to
  handle the deferred ops as a separate data structure.
- Various fixes and cleanups.
-----BEGIN PGP SIGNATURE-----

iQIzBAABCgAdFiEEUzaAxoMeQq6m2jMV+H93GTRKtOsFAltsgIkACgkQ+H93GTRK
tOvWpw//aLqxI+tPar9gHJdz8GvT7AMzk8ICyFovLThlhIYQicVfsyKsl1fYavmX
r1yDqULczYOS1OU+l0etvAXoKJEBy1wwe/xdC+eRv/3xhI4mpXO0vXH27rOrVdUa
kyrVHxXDJHyBb8uwmNnr44V2TNt/FO1nsMAnqNlwuklXnmPxbA0FKfLJeICylmKZ
eMDYkFN+9Swx0MLSwKaEOvuOTohlqOlED9f9/ews8cM1nb/qgIwMFcAhJG8ZoxR+
mK2buCbQQtyuyrZTVMtEoEgzU61aV82UUqe0HfKdeo3DG9S1jAPi0kj69riObHV3
YQO3pCCG5OJ0UiLWziOPKgCw7XEH9LrRvwbMIXFW7B6kd8y+ASwFYPgIz31oKOm0
CbmyqdLugZJ0BWQsi5VawvRriJL3tpZYs1mVdOQtA8FXJSxipyDywjdG3EwcSs0X
lPAewFLPka4R+xhvpsTIcnWp3+2kysxI7jO4OjvTxy1+0UYnrVTnmqd3F/GQrSdR
TYxv4d060JEKM3RUuG5Grr4P1BJ+63fAEZCrW8STXMueaXcA4r6hhLjZG/e2m9VI
IdDjkXE9CxHs8CP+DWaoO+FYOs6CWZYYQIcIJBaIL/mYXmlqbnKCHrb71JWN6FGU
0vQCtwNBbJIp2BTrGO5ShF6WviJ5KxxDhZVdD/Ul7Vh07vsoxBw=
=4oGw
-----END PGP SIGNATURE-----